Multipath I/O (MPIO) is a feature which provides a host with the ability to utilize multiple physical paths to a data storage array. In particular, if the host is unable to perform an I/O operation on the data storage array through one physical path, the host is able to retry the I/O operation on that array through another physical path. PowerPath® which is offered by EMC Corporation of Hopkinton, Mass. is an example of a multipathing software product.
After a data storage array has been in use for a period of time, the owner of the array may wish to replace that original array with a newer array, i.e., to migrate from the original array to a replacement array perhaps with more capacity, faster processors, newer components, additional features, etc. Open Replicator for Symmetrix (ORS), which is offered by EMC Corporation, is an example of a software product which facilitates creation of point-in-time copies of data to enable effective data migration from an original array to a replacement array while a host maintains online access to host data, i.e., online data migration. Another example is Symmetrix Remote Data Facility (SRDF) which is also offered by EMC Corporation. There are other replication software products available as well.
One conventional approach to online data migration involves making the replacement array available to a host even though some or all of the host data may have not yet been transferred to the replacement array from the original array. That is, the replacement array starts copying the host data from the original array (i.e., a background copy task), but behaves to the host as if all of the host data already resides on the replacement array. Along these lines, if the replacement array receives a host I/O request for particular host data that has not yet been copied from the original array, the replacement array immediately copies that host data in response to the I/O request, i.e., a copy-on-demand operation. Once the replacement array receives the requested host data from the original array, the replacement array provides that host data to the host as well as stores that host data thereafter. This process of “hot pulling” host data from the original array in response to host I/O requests can continue in conjunction with standard background data copying until all of the host data has been copied from the original array to the replacement array.